1. Field of the Invention
The present invention relates to a wafer sheet, a method of producing a semiconductor device using the wafer sheet and an apparatus for producing a semiconductor device. More particularly, it relates to a semiconductor wafer laminated on its back side to a wafer sheet which is covered by thermosetting adhesive layers on both sides, while element separation and die bonding operations are carried out substantially simultaneously, thereby improving the productivity, a method of producing a semiconductor device using the wafer sheet and an apparatus for producing a semiconductor device.
2. Description of the Related Art
Steps for producing semiconductor devices include a bonding step. The bonding step comprises a die bonding step where each of semiconductor elements (dice) which have been separated from a wafer is bonded onto a die pad and a wire bonding step where electrodes on the semiconductor element 3 and inner leads are electrically connected with each other by means of thin metal wires.
As the wire bonding operation, among the operations in the bonding step, has been increasingly automated, it has become relatively more important to rationalize the feeding of semiconductor elements around a die bonder. As a result, various methods of feeding the semiconductor elements, such as a direct pickup system, have recently been devised by manufacturers and have been disseminating.
FIGS. 10A, 10B are schematic diagrams showing a die bonding apparatus with the direct pickup system of the prior art, where FIG. 10A is a perspective view and FIG. 10B is a cross sectional view of a key portion.
As shown in the drawing, while locating the semiconductor element 3 attached to the wafer sheet 2, only good elements are thrust up by a thrust pin 38, picked up by a suction head 39, moved onto a die pad 30 of a lead frame 29 and bonded thereon. Good elements may be identified either by using an ink mark provided on the surface of a defective element or by means of a map of good elements stored on a floppy disk (not shown and hereinafter referred to as F/D) which provides the information on the positions of good elements.
However, either of the methods described above has such problems that, since the selected elements are thrust up by the thrust pin 38, picked up by the suction head 39, moved onto the die pad 30 which is located at a distance and bonded thereon, there is a possibility that the semiconductor element 3 is damaged during thrusting, picking up or die bonding, and the throughput decreases due to longer time required for the bonding step as a whole. In an attempt to solve the problems of the prior art, for example, Japanese Patent Laid-Open Publication No. Hei. 6-204267 discloses a semiconductor wafer having an adhesive tape attached on the front side thereof is diced on the back side thereof and each of the separated semiconductor elements is bonded on the back side to a element mounting position of a lead frame by pressing on the front surface of the semiconductor element via the adhesive tape using a die bonding fixture, and the adhesive tape is removed and the semiconductor element is bonded on the back side to the element mounting position in a single step.
However, with the die bonding step described above, although the adhesive tape is attached to the front surface of the semiconductor wafer, possibility of damaging the integrated circuit due to mechanical impact cannot be eliminated because the front surface of the semiconductor element the integrated circuit is formed is pressed via the adhesive tape by the die bonding fixture. Further, since the adhesive tape is attached to the front surface of the semiconductor wafer, an adhesive material sticks onto the surface of electrodes provided on the front surface of the wafer thereby causing contamination and, wire bonding of thin metal wires thereon without cleaning may result in poor reliability of connection. To solve this problem, it is required to introduce a new process of cleaning the electrode surface.